We measured intra- and extra-cellular concentrations of oxygen in Mouse Monocyte-Macrophage cultures, J774A.1, using the Ru(II) complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+with the fluorescence lifetime microscope. Using the time-resolved technique we were able to compare response for oxygen concentrations between air and nitrogen atmosphere. The oxygen-dependent changes in phase angle for [Ru(bpy)3]2+ and [Ru(phen)3]2+ inside and outside cells were in the order of 10-12 degrees (400 kHz) between the air and nitrogen. The spatial resolution under the microscope was at the diffraction limit, which allowed us to measure the concentration of oxygen inside a single 12 micrometer size macrophage.